Accessing a Gmail Account from Nextcloud
Due to Google's security policies, accessing your Gmail account from Nextcloud requires additional steps.
If you use two factor authentication, you'll need to generate an app password:
Visit https://myaccount.google.com/apppasswords from a web browser
At the bottom of the page, click the drop-down box labeled "Select app"
Choose the option, "Other (Custom name)"
Enter a descriptive name, such as "Nextcloud Mail"
Click "Generate"
Go back to Nextcloud (Mail/Rainloop), and enter your e-mail address and the app password you just generated
Your Gmail account should now be accessible from within Nextcloud
If you are not using two factor authentication, you'll need to allow "Less Secure Apps":
Visit https://myaccount.google.com/lesssecureapps
Toggle the radio button "Allow less secure apps" to the "ON" position
Go back to Nextcloud (Mail/Rainloop), and enter your e-mail address and Google password
Your Gmail account should now be accessible from within Nextcloud